1. Field of the Invention
The present invention relates to a passive matrix or active matrix display such as a liquid crystal display and, more particularly, to a fashionable display device in which the ratio of the area of the display portion to the area of the substrates of the display device is increased by effectively mounting a driver semiconductor integrated circuit.
2. Description of the Related Art
Passive matrix type and active matrix type constructions are known as matrix display devices. In the passive matrix type, a number of stripe-shaped conducting lines (row lines) made of a transparent conductive film or the like are arrayed in a certain direction on a first substrate. On a second substrate, similar stripe-shaped conducting lines (column lines) are arrayed in a direction substantially perpendicular to the conducting lines on the first substrate. Both substrates are so arranged that the conducting lines on them intersect each other.
An electrooptical material such as a liquid crystal material whose transparency, reflectivity, or scattering performance is varied by a voltage, current, or the like is positioned between both substrates. If a voltage or current is applied between an addressed row line on the first substrate and an addressed column line on the second substrate, then the transparency, reflectivity, or scattering performance at the intersection can be set to a desired value. In this way, the display device can be matrix driven.
In the active matrix construction, row and column lines are formed on the first substrate by multilayer metallization techniques. Pixel electrodes are formed at the intersections of the row and column lines. An active device such as a thin-film transistor (TFT) is formed at each pixel electrode to control the potential or current in the pixel electrode. A transparent conductive film is also formed on the second substrate. Both substrates are so arranged that the pixel electrodes on the first substrate are located opposite to the transparent conductive film on the second substrate.
In either type, the substrates are selected according to the used process. For example, the passive matrix construction needs no complex process steps except for steps where the transparent conductive films are formed and etched into row and column conducting line patterns. The substrates of this passive matrix type may be made from plastic, as well as from glass. On the other hand, to manufacture the active matrix construction, a relatively high-temperature film formation step is required. Furthermore, the active matrix type must keep out mobile ions such as sodium ions. The substrates of the active matrix type must be made of glass containing a quite low concentration of alkali.
In any type of prior art matrix display device excluding special constructions, a semiconductor integrated circuit (peripheral driver circuit) for driving the matrix is required to be mounted. In the past, this has been done by tape automated bonding (TAB) or chip on glass (COG). However, the matrix construction contains as many as several hundreds of rows. Therefore, the integrated circuit has a very large number of terminals. The corresponding driver circuit takes the form of a rectangular IC package or semiconductor chip. To connect these terminals with the conducting lines on the substrates, it is necessary to lay the conducting lines in a complex manner. As a consequence, the ratio of the area of the peripheral portion, or non-display portion, to the area of the display portion is not negligibly small.
A method for solving this problem is disclosed in Japanese Patent Laid-Open No. 14880/1995, and consisting of forming a driver circuit on an elongated substrate (referred to as a stick or stick crystal) having a length substantially equal to one side of the matrix construction and connecting the driver circuit with the terminal portion of the matrix. This arrangement is permitted, because a width of about 2 mm suffices for the driver circuit. Therefore, almost the whole area of the substrate can be made a viewing screen.
Of course, in this case, where the matrix has a large area, it is impossible to form a circuit on a silicon wafer. Consequently, it is necessary to form the circuit on a glass substrate or the like. Hence, active devices used in semiconductor devices are TFTs.
However, where a stick crystal is employed, the thickness of the substrate of the driver circuit has been an obstacle to miniaturization of the whole display device. For example, where the display device should be made thinner, the thickness of the substrate is allowed to be set to 0.3 mm by optimising the kind of the substrate and the manufacturing steps. However, because of the strength necessary during manufacturing steps, it is difficult to reduce the thickness of the stick crystal below 0.5 mm. As a result, where two substrates are bonded together, the stick crystal protrudes as long as 0.2 mm or more.
Furthermore, if the stick crystal differs from the substrates of the display device in kind, then defects may be produced in the circuit because of the difference in coefficient of thermal expansion and for other causes. Especially, where a plastic substrate is used in a display device, this problem is conspicuous, because poor heat resistance of plastics makes it substantially impossible to use a plastic substrate as a stick crystal substrate.
Moreover, where the kind of the substrate supporting the stick crystal is different from the kind of the substrates of the display device, other known methods are used to circumvent the above-described problem. In one known method, a semiconductor integrated circuit having TFTs is fabricated on other support substrate. Then, the circuit is peeled off and adhesively bonded to another substrate. In another known method, the original support substrate is removed after adhesively bonding the circuit to another substrate. This technique is generally known as silicon-on-insulator (SOI) technique.
However, when the support substrate is removed, the semiconductor integrated circuit is often damaged, thus deteriorating the manufacturing yield.